Dual-mode shaped charge device

ABSTRACT

An explosive device composed of an explosive annular shaped charge liner having a given configuration; and an initiator configured to detonate said liner according to a selected one of two different jet formation modes.

This provisional patent application describes an explosively driven annular shaped charge liner having a unique shape together with an initiator design to provide two different jet formation modes from a single liner configuration. These modes include formation of a “tubular” jet or alternatively, a “constituted” jet. The reconstituted jet is created when the tubular jet is designed to have significant convergence (inward radial and forward velocity components) and upon collision with the central axis of the device. This forms a new jet that is co-aligned with the axis as a solid massive jet with properties similar to jets formed from a hemispherical liner. There have been various attempts in the past to produce tubular jets or ring projectiles by several researchers [1, 2, 3]. For example, Glass, Kronman, and Golaski [1] at the Ballistic Research Laboratory in 1969, reported work on an annular charge with a conical cross section to form a “cookie cutter” jet. Results indicated that tubular jets could be formed but these had some divergence and poor standoff performance. As far as is known, there have been no attempts to form a solid jet on-axis from the motion of a previously formed jet (converging conical shaped jet). In this regard, the reconstituted jet formation presented herein represents a new jetting mechanism.

[ML1]

The weapons community has a long-standing commitment to pursuit of advanced shaped charge designs as well as experimental and analytical descriptions of their performance. The present dual-mode concept is vastly aided by today's electronics that can be used to control initiation sites, making this shaped charge a candidate for a single geometry, multi-mode warhead system. Such a system now appears to have come into its own using precision initiators and electronics that can control such initiators. Further, modern day computational capability (FE analysis) can provide design guidance on the involved complex jet formation mechanics. For this purpose the ALEGRA code was used. The following sections describe liner designs, initiation system, and warhead function related to producing either tubular or constituted jets.

CHARGE DESIGN

FIG. 1. Schematic of the Annular Shaped Charge design depicting liner shape, position, and taper, along with explosive charge and initiator system.

FIG. 2. ALEGRA calculations at various stages of initiation, detonation, liner collapse, and jet formation: a) upper—tubular jet design and b) lower—constituted jet design.

FIG. 3. Proposed Dual-mode warhead device that includes multiple initiators (A & B) for varying the initiation radius of the device.

LINER CONFIGURATION/INITIATION SCHEME

The present approach uses liner geometry of annular form wherein the cross section of the liner has hemispherical, or semicircular shape, although other shapes are possible. The configuration is shown in FIG. 1. Here, the liner shape may be described as a hemi-toroidal shell with the central axis of its explosive charge aligned with the axis of the toroid. At least two variations are possible; one with a uniform liner thickness (lower dotted lines) and another that had a tapered liner (shown in upper half). For the liner having uniform thickness, parameter A=0.

The charge consisted of high explosives (HE) such as 75/25 OCTOL. Typical length-to-diameter (L/D) ratio is ½, although other ratios are possible. As such, the configuration represents a compact design. The liner thickness could be composed of various metals consistent with shaped charges to include copper, tantalum, aluminum, iron, or molybdenum, for example. Starting liner thickness could be from 2 to 4 percent of the charge diameter, although other thicknesses could be used as well, based on previous experience (Grace et al.) The computational physics code, ALEGRA, was used to demonstrate function of this device in both modes as shown in FIG. 2.

The initiator consisted of a relatively “thick” PETN based explosive sheet disk (Primasheet), which was centered by a machined polyethylene part that also centered the AR211 detonator on a PBX booster pellet. Upon detonator activation, a circular detonation wave will proceed outward radially until it interacts with and initiates the OCTOL of the risers. The inner body of explosives is protected from initiation by a polyethylene wave shaper (WS). The risers start the detonation process in the OCTOL, which is propagated into the main body of explosives. The initiation system generates a “ring” detonation wave front, having annular shape. The effective radius of initiation, I₀, is defined at the center of the riser.

A hemispherical cross section was chosen for the annular liner of the hemispherical shape since this shape is more robust with respect to off-axis initiation. When the wave is tilted, initially the wave front “sees” symmetry of the spherical liner and hence, a quality jet can be formed. The initiation scheme to produce a tubular jet is designed so that the ensuing detonation front will have annular symmetry (ring) so as to strike the liner simultaneously about the pole region. However, the initiation radius was adjusted somewhat to produce a jet having tubular shaped walls that travel parallel to the charge axis without convergence or divergence.

A constituted jet is produced when the charge is initiated at a radius much greater than the pole radius. In this case, the jet formed is directed both forward and inward in the form of a flowing converging cone of jet material. When such converging jet strikes the axis, it creates a newly formed or constituted on-axis jet in solid form.

Computational Details

The ALEGRA 2-D Shock Wave Physics code [5] was exercised to calculate jet formation. The modeling has axial symmetry, on a mesh that extends 0.4 m in the axial direction and 0.15 m in the radial direction. The mesh elements are 0.5 mm by 0.5 mm squares in a fine mesh region that starts at the base of the main charge and extends forward, and from the axis to a radius of 60 mm; beyond the fine mesh region, the elements increase in size linearly to 2 mm as they reach the edge of the mesh. Jet formation takes place in the fine mesh region—the course mesh exists only to accommodate expansion of explosive gasses.

The ALEGRA library HVRB model was used for OCTOL, while the detonator and Primasheet material were taken from the ALEGRA library JWL model “C-4”. The wave shaper and other plastic parts use the ALEGRA library sesame EOS for polyethylene with the Johnson Cook elastic plastic model for Lexan. The copper liner uses the ALEGRA library sesame EOS for copper with the Zerilli-Armstrong elastic-plastic model. All void space around the test object is filled with the ALEGRA sesame model for dry air.

FIG. 1 defines parameters explored with the following physical dimensions of the charge/liner/initiator assembly, i.e., 1) tapered vs. uniform wall liners, and 2) initiation radius. All charges had D=101.6 mm and L=50.8 mm, outer liner radius R=19.6 mm, liner pole radius was P=25.4 mm, B=50.8 mm, and C=6.35 mm.

FIG. 2 shows the annular lined charge using ALEGRA along with a typical calculation of tubular and constituted jet formation based on a single liner configuration but with two different initiation radii, I₀.

DISCUSSION AND SUMMARY

This application deals with a dual-mode warhead as a wall breach device cutting large holes in targets (tubular jets) or as a solid on-axis jetting device (constituted jet) for deep penetration similar to conventional shaped charges. We propose that an overhead attack or wall breach warhead could be designed having the annular lined charge geometry investigated herein. A possible warhead configuration is shown in FIG. 3, where two separate initiators are included. Using present-day advanced electronics, the initiation (detonator A or B) can be selected by the gunner before weapon firing or conditionally “in flight” as target information is acquired. The charge could possibly fit to a tandem device, where if the target were concrete or brick, a large hole would be desired as a precursor for a follow-through munition, for example. In this case, detonator A would be activated to generate a tubular jet. If the target were to be a hardened or armored structure, where deep penetration is required, then detonator B would be activated. Thus, the warhead can function in two separate but distinct modes (dual mode) on command and provide appropriate and desired effects on target in either case. Even in a single mode of peripheral initiation, the warhead can serve as a more effective fly-over/shoot down or top attack device. The enhanced velocity of the reconstituted jet compared to an EFP (5.3 km/s vs. 2.5 km/s), can provide higher kinetic energy effects on target while avoiding detrimental crossing velocity effects.

It is believed that for the first time, a first formed jet is being used as a moving liner to form a second or reconstituted new jet. Thus in that regard, the authors are patenting a totally new jetting mechanism for shaped charges.

REFERENCES

-   1. Glass, C. M., Kronman, S., and Golaski, S. K., “The Cookie Cutter     Warhead,” US Army Ballistic Research Laboratory Report No. 1455,     October 1969. -   2. Liedel, D. J., “A Design of an Annular-Jet Charge for Explosive     Cutting,” Doctoral Dissertation, Drexel University, Philadelphia,     Pa., June 1978. -   3. Fong, R., L. Thompson, W. Ng, “Toriodal Warhead Development,” in     Proceedings 25^(th) Int. Symp. on Ballistics, Beijing, China, 2005. -   4. Grace, F. I., S. K. Golaski, B. R. Scott, “The Nature of Jets     from Hemispherical Lined Shaped Charges,” in Proceedings 8^(th) Int.     Symp. on Ballistics, Orlando, Fla., October 1984. -   5. Robinson, A. C., et al, ALEGRA Users Manual, Sandia National     Laboratory, SAND2014-1236, 2014. 

What is claimed is:
 1. An explosive device comprising: an explosive annular shaped charge liner having a given configuration; and an initiator configured to detonate said liner according to a selected one of two different jet formation modes. 